This invention pertains to the field of controlling devices for electric motors, or for any other devices requiring a non-linear response to a linear mechanical control input. More specifically, it pertains to improved photoelectric means for translating accelerator pedal movement into changes in motor speed for electric vehicles.
It has long been known that one method of controlling the speed of a DC motor is to operate the motor in a configuration in which the armature, or rotating portion, of the motor is connected directly to the power source, and the field, or stationary portion, is separately excited to provide speed control. As the field excitation is decreased the motor gains speed. This method, called "field control", is being increasingly used in electric vehicles, either alone or in combination with some form of armature control. Field control has the advantage that field currents are relatively small compared to the armature currents, thus they are easier to control. Field controllers are less expensive and dissipate less heat than armature controllers.
A problem arises in using a field controlled motor in an electric vehicle in that the response of the motor to a change in field excitation is highly non-linear (See FIG. 2). If a linear device, such as a conventional potentiometer, is used in the control circuit, the first half of the accelerator pedal travel produces little change in motor speed, and the second half produces a far greater response. This produces a very unnatural control feeling for the driver. Nevertheless, the potentiometer, or a similar variable resistance element (i.e. a compression resistor as in Schenkelberger, U.S. Pat. No. 3,114,871), has in the past been the standard device used on speed controls in electric vehicles (see, e.g., Thompson, U.S. Pat. No. 3,989,990).
The non-linearity problem can be overcome to an extent through the use of a custom non-linear potentiometer, but these tend to be expensive. Moreover, one skilled in the art will recognize that potentiometers are prone to wear with use. Most commonly used potentiometers use a carbon element which wears badly with time, creating "noise" on the signal and erratic response before failing. In the rugged environment of a motor vehicle they tend to accumulate dirt and moisture, which creates yet more "noise" and accelerates failure still more.
Some controllers have incorporated magnetic-sensitive devices such as Hall-effect semiconductors (Berman, U.S. Pat. No. 3,818,292; Elliott, U.S. Pat. No. 3,562,616) or variable transformers (Truemper, et al, U.S. Pat. No. 3,469,164) as the variable element. These may solve the linearity problem, but tend to expensive and complicated in operation. Moreover, the Hall effect devices can be sensitive to stray magnetic fields and variations in temperature, as are commonly found in the environment of an electric vehicle.
The use of a photoelectric detector coupled with a light source to effect changes in motor speed has been known for many years. Stoekle, U.S. Pat. No. 1,582,702, was typical of these methods of photoelectric control, using a shaded screen between the photodetector and the light source. Alternatively, an iris or shutter may be interposed in place of the screen, as in the Singer sewing machine control disclosed in Bostrom, U.S. Pat. No. 3,145,304. The non-linear response desired may be derived, though not easily changed, by mechanical linkages or suitable shading of the screen. Although a step forward, these systems are still prone to problems from the dirt, moisture, and vibration inherent in motor vehicle operation, and can be complicated and expensive to build.
Thus, it is an object of this invention to provide an improved accelerator control for electric motors.
It is a further object of the invention to provide an accelerator control for electric vehicles whose output characteristics may be varied to provide a linear pedal position to motor speed response curve.
It is a further object of the invention to provide an accelerator control which is rugged, simple, and relatively impervious to the vibration, noise, moisture, and dirt of a vehicular environment.
It is a still further object of the invention to provide an accelerator control for electric vehicles which can be used with a minimum of additional parts or external wiring.
Other objects of the invention will become obvious to one skilled in the art as this disclosure progresses.